1. Field of Application
The present invention relates to a transceiver for use in a communication system in which a transmission code is constituted by a PWM (pulse width modulation) signal.
2. Background Technology
Various types of communication system for installation on a vehicle are known, such as the CAN system, LIN system, etc., which employ a data bus as a communication path. Such systems are described for example in “Comprehensive Description of Vehicle-installation Network Systems” by M. Sato, CQ Publishing Co., (Japan), Dec. 1, 2005.
With such a communication system, to ensure efficient communication, it is desirable to mutually synchronize the operations of transceivers which are located at respective nodes of the system and which transmit/receive signals via the communication path.
One method of achieving such synchronization is as follows. A specific one of the nodes (i.e., the transceiver of that node) transmits a signal to the communication path, which is encoded with a transmission code containing a clock signal component. Each of the other nodes receives that signal via the communication path, extracts the clock signal component, generates a local clock signal (by frequency division, etc.) using the clock signal component as a timing reference, and synchronizes transmission operations with the received clock signal component.
One known type of such a transmission code utilizes PWM signals modulated with two different duty factors, for respectively expressing 1 and 0 bit values. Specifically, each bit is transmitted as a first (binary level) transition, at a bit boundary, followed by an opposite-direction transition. Thus the waveform of the signal expressing the transmission code has edges which occur respectively at a bit boundary and within the bit interval.
To suppress generation of noise on the communication path, each of these edges should have a gradual slope. However in the prior art, to achieve an increased data transmission rate it is necessary for the slope of each the edges to be made more steep, i.e., the steeper the slope, the shorter can become the interval between successive bits. However the steeper the edge slope, the greater will be the amount of noise produced on the communication path. Thus there are conflicting requirements for the edge slope, with respect to achieving a high speed of communication via the communication path while ensuring a low level of noise on the communication path.
In addition, as illustrated in the waveform diagrams of FIG. 7, a problem of unstable operation may arise if, with a specific degree of steepness of the edge slope, the interval between successive edges of the signal waveform expressing the transmission code is made excessively short, so that a signal overlap condition occurs on the communication path.
Thus there is a problem that it is difficult to achieve a satisfactory trade-off between a degree of steepness of edge slope which enables sufficient control of noise and a degree which enables increased communication speed.